1. Field of the Invention
The present invention relates to quinoxaline derivatives. The present invention also relates to light-emitting elements, light-emitting devices, and electronic devices using the quinoxaline derivatives.
2. Description of the Related Art
An organic compound can have various structures in comparison with an inorganic compound, and it is possible to synthesize materials having various functions by appropriate molecular design. Owing to these advantages, photo electronics and electronics which utilize functional organic materials have been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like can be given as examples of electronic devices utilizing an organic compound as a functional material. These devices take advantage of electrical properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable progress.
It is considered that a light-emitting mechanism of a light-emitting element is as follows: when a voltage is applied to a pair of electrodes between which a light-emitting layer is interposed, electrons injected from a cathode and holes injected from an anode are recombined at luminescent centers in the light-emitting layer to form molecular excitons, and the molecular excitons release energy to cause light emission when relaxing to the ground state. Singlet excitation and triplet excitation are known as excitation states, and it is considered that luminescence can be conducted through either one of those excitation states.
In an attempt to improve performance of such a light-emitting element, there are many problems which depend on the material, and in order to solve these problems, improvement of element structure, development of a material, and the like have been carried out.
For example, as a material with an electron-transporting property for a light-emitting element, tris(8-quinolinolato)aluminum(III) (abbr.: Alq) is widely used (see Reference 1: Taishi TSUJI et al., SID 04 DIGEST, 35, PP. 900-903 (2004)). However, development of a material with more superior characteristics such as further higher mobility has been demanded. In particular, in view of commercialization, reduction in power consumption is an important object, and developments of material and light-emitting element with more superior characteristics have been desired.